Basic MRI I

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# Basic MRI I - PowerPoint PPT Presentation

Basic MRI I. Chapter 2. The Magnet. The heart of the system Induces tissue magnetization Induces resonance with a specific frequency Frequency is proportional to field strength. Frequency is proportional to field strength. Freq = gyromagnetic ratio x field strength

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Presentation Transcript

### Basic MRI I

Chapter 2

The Magnet
• The heart of the system
• Induces tissue magnetization
• Induces resonance with a specific frequency
• Frequency is proportional to field strength
Frequency is proportional to field strength
• Freq = gyromagnetic ratio x field strength
• Freq = “gamma” x B-naught
• Freq = γHB0
• Gyromagnetic ratio = γH = 43.58 MHz/T
• Field strength = B0
Magnetic field
• Magnetic fields have strength and direction – they are vectors
• Strength measured in Gauss or in Tesla
• 1 Tesla = 1 T = 10,000 Gauss = 10,000 G
• 1000 G = 1 kG
• Direction is defined as the “z-direction” in MR, in 3D Cartesian coordinates
• Orientation
• Definition of orthogonal = “ at right angles”
• Functions
• To get images—all types
• To get specialized images (gradient echo)
• To reduce artifacts
• Strength
• Units of mT/m
• High value = ~ 20 mT/m
• Rise time = time for gradient to reach its maximum, like 0.20 msec
• Slew rate = rate of gradient change, like 100 mT/m/msec
Eddy currents
• Electrical currents induced or generated in conducting materials, in metal in the machine, minimized by gradients
Shimming
• To achieve homogeneous field strength
Shielding
• Passive—large pieces of ferromagnetic metal like iron
• Active—electromagnets to decrease the extent of the field
RF system
• Coils
• Body coils
• Surface coils
• Special coils
• TMJ